ASSESSMENT OF BREAST DENSITY AND RELATED CANCER RISK
A method for assessing breast density executed at least in part by a computer system, identifies breast tissue from the electronic image data for at least one mammographic image, then performs an initial segmentation of fibroglandular tissue within the breast tissue according to at least one of gradient and uniformity data that is derived from the image data. The initial segmentation is refined using a pixel clustering process. A localized segmentation is obtained from the refined segmentation by generating and combining a density probability mapping and a homogeneity mapping from the image data. A percent density value for the at least one image is calculated and stored in a memory.
Reference is made to, and priority is claimed from, U.S. Ser. No. 61/116,047, filed as a provisional patent application on Nov. 19, 2008, entitled “Assessment Of Breast Density And Related Cancer Risk”, in the names of Zhimin Huo et al., and which is commonly assigned.
FIELD OF THE INVENTIONThe invention generally relates to image processing and analysis and computer-aided diagnosis (CAD) and more particularly relates to methods that assess and use data related to the density of breast tissue as a risk factor in breast cancer diagnosis.
BACKGROUND OF THE INVENTIONIn a number of studies, breast density has been found to be a factor for assessing cancer risk. Among factors that determine density is the relative proportion of dense to fatty tissues, sometimes expressed as mammographic percent density, or MPD. The average breast generally has about 50% fibroglandular tissue, a mixture of fibrous connective tissue and the glandular epithelial cells that line the ducts of the breast (the parenchyma), and 50% fat tissue. The radiological appearance of the breast varies between individuals, in part, because of variations in the relative amounts of fatty and fibroglandular tissue. Since fat has a lower effective atomic number than that of fibroglandular tissue, there is less x-ray attenuation in fatty tissue than in fibroglandular tissue. Fat appears dark (i.e., has a higher optical density) on a mammogram, while fibroglandular tissue appears light (i.e., exhibits a lower optical density). Regions of brightness associated with fibroglandular tissue are normally considered by diagnosticians to have increased “mammographic density”. It is known that mammographic imaging techniques are less successful with denser breast tissue than with predominantly fat tissue. Fibroglandular tissue in the breast tends to attenuate x-rays to a greater degree than does fat tissue, leading to increased difficulty in detection of cancer sites for denser breasts.
Assessment of breast density has been acknowledged to be useful for effective mammogram interpretation. As a guideline for classification, the American College of Radiology (ACR) Breast Imaging Reporting and Data System (BIRADS) has identified four major groupings for breast tissue density. Class I corresponds to breasts having high concentration of fat tissue. The Class II grouping indicates scattered fibroglandular densities. Class III indicates heterogeneously dense tissue. Class IV corresponds to extremely high breast density.
Women with increased mammographic parenchymal density can have four- to six times the risk over women with primarily fatty breasts. Some believe that increased density may indicate a relatively higher amount of tissue at risk for developing breast cancer. Since most breast cancers develop from the epithelial cells that line the ducts of the breast, having more of this tissue as reflected by increased mammographic density may indicate higher likelihood of developing breast cancer. In addition, some studies indicate that lesions in higher density areas are themselves more difficult to detect from the mammogram than are lesions in fatty regions, somewhat compounding the problem. Increase in density over time can also be an indicator of a disease condition.
Saha et al. in an article entitled “Breast tissue density quantification via digitized mammograms”, IEEE Transactions on Medical Imaging, vol. 20, no. 8, 2001) describes a scale-based fuzzy connectivity method to extract dense tissue regions from mammographic image; a comparison between segmentation in craniocaudal (CC) and mediolateral-oblique (MLO) mammographic views showed a strong correlation. Carri et al. in “A new method for quantitative analysis of mammographic density” (Medical Physics, 34(11), November 2007) propose a method of segmenting dense tissue from mammography using K-mean tissue clustering technique. Ferrari et al. in “Segmentation of the fibro-glandular disc in mammograms via Gaussian mixture modeling” (Med. Biol. Eng. Comput., vol. 42, pp. 378-387, 2004) used expectation maximization in combination with a minimum description length to provide the parameters for a mixture of four Gaussians. The statistical model was used to segment the fibroglandular disk, and a quantitative evaluation was provided. Selvan et al. in “Parameter estimation in stochastic mammogram model by heuristic optimization techniques” (IEEE Trans. Inf. Technol. Biomed., vol. 10, no. 4, pp. 685-695, 2006) used a heuristic optimization approach to estimate model parameters for a larger number of regions. Initial segmentation results were assessed by radiologists and showed improvement when compared to alternative approaches.
Still other approaches for distinguishing dense from fatty tissue using texture-based discrimination between tissue types according to spatial gray-level dependency matrices. Other researchers have developed segmentation techniques using a set of co-occurrence matrices and using the resulting density classification to compute the relative area of the density regions as the feature space.
While various methods may have achieved some level of success in segmenting and identifying areas of different density in the mammography image, however, there is acknowledged to be considerable room for improvement in density detection, display, and reporting. Moreover, although tissue density has been recognized as a significant factor for risk assessment, conventional mammography CAD systems have not utilized this information to help obtain improved results from diagnostic tools. Information relating to breast density has not been provided in any standard way, but must be obtained subjectively or must be calculated independently from the mammography image itself.
Applicants believe that, overall, obtaining and using tissue density information from the mammography image can help to manage patient care, to increase the effectiveness and value of imaging and image processing equipment, and to provide the diagnostician with a more uniform metric for describing and evaluating breast density.
SUMMARY OF THE INVENTIONIt is an object of the present invention to advance the art of computer-aided diagnosis for mammography and other tissue imaging. With this object in mind, the present invention provides a method for assessing breast density executed at least in part by a computer system, the method comprising: identifying breast tissue from the electronic image data for at least one mammographic image; performing an initial segmentation of fibroglandular tissue within the breast tissue according to at least one of gradient and uniformity data that is derived from the image data; generating a refined segmentation of the fibroglandular tissue within the breast tissue by refining the initial segmentation using a pixel clustering process; obtaining a localized segmentation from the refined segmentation by generating and combining a density probability mapping and a homogeneity mapping from the image data; and calculating a percent density value for the at least one image and storing the percent density value in a memory.
It is a feature of the present invention that it evaluates breast tissue density using both global and local image data in successive processing steps. This helps to avoid a condition in which the solution becomes trapped in a local minimum or maximum and helps to provide improved local and global results.
It is an advantage of the present invention that it is relatively insensitive to differences in image contrast or other quality characteristics or to differences due to the specific type of radiology system used for obtaining the image.
These objects are given only by way of illustrative example, and such objects may be exemplary of one or more embodiments of the invention. Other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. The invention is defined by the appended claims.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other.
The following is a detailed description of the preferred embodiments of the invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures.
Reference is also made to commonly assigned U.S. patent application Ser. No. 11/616,953 filed 28 Dec. 2006 and entitled “Method for Classifying Breast Tissue Density” by Luo et al.
For the detailed description that follows, the mammographic image is defined as f(X), where X denotes the pixel array and f(x) denotes the intensity value for pixel x in X.
In the context of the present disclosure, the term “dense tissue” is generally considered synonomous with fibroglandular tissue of the breast. Within the mammography image, this dense tissue is readily distinguishable from fatty tissue to those skilled in breast cancer diagnosis.
The logic flow diagram of
Processing using the sequence shown in
Still referring to the process of
Tissue clustering in step 1300 yields the dense membership map of binary clustering results 1504 or 1506 (
A local fibroglandular tissue segmentation step 1400 then performs further segmentation using features based on local variation and density spatial relationships and applying feature-based clustering. This generates a binary segmentation, shown overlaid against the original in an overlay image 1516 (
The selection of an initial threshold that separates dense from fatty tissue is based on the observation that tissue within either the dense tissue region or the fatty tissue region is relatively homogeneous. The boundary between dense and fatty tissue contains most of the shape information, usually measured by the gradients of the points along the boundary. The desired threshold is one that can separate dense tissue from fatty tissue with a maximum gradient along the boundary and minimize the intensity variation within both tissue types. At the same time, this threshold value maximizes the variation between dense tissue and fatty tissue. Since it is difficult to calculate a single threshold t that both maximizes the gradient and maximizes inter-tissue variation, two interim thresholds t1 and t2, are first estimated, then used to calculate threshold t. The interim thresholds t1 and t2, and the calculated threshold t for this initial segmentation processing are defined using the following sequence in one embodiment:
1. Convert image data from 12-bit to 8-bit format. This generates a reduced-resolution grayscale image and simplifies subsequent computation.
2. Search the threshold t1 that gives the maximum uniformity within each of the two regions separated by the threshold t1.
3. Search the gray value t2 that gives the maximum normalized gradient for pixels with a gray value of t2.
4. Determine the resulting threshold t for initial membership assignment based on the values of t1 and t2. This can include finding the average of values t1 and t2, for example.
In step 2, uniformity measurement is used to select a threshold, t1, that maximizes the computed homogeneity or uniformity within each tissue type. The uniformity of a feature (for example, its gray value) over a region is inversely proportional to the variance of the values of that feature, evaluated at every pixel belonging to that region. The lower this variance, the higher the uniformity.
With an image segmented into two regions, fatty and dense by the computed resulting threshold t, the uniformity measurement U(t) at the threshold t is defined as:
where σi is the standard deviation of pixel intensities belonging to a respective region ri; Cont1 is a positive normalization constant. Using this computation, the threshold t1 that gives the highest uniformity is searched sequentially for each of the 8-bit gray levels, from lowest (L) to highest (H).
For step 3 as given earlier, gradient analysis then yields a type of shape measurement that can be used to select another threshold, t2, relating the edges between two tissue types. A shape measurement, G(k), can be defined as a normalized gradient from all the pixels whose gray value is k. Threshold t2 is then determined using equation (3) to search each gray level in sequence, from low to high.
In general, values t1 obtained from (2) and t2 obtained from (3) are not the same. It would be best to have an image segmented at an ideal threshold value t such that, after the threshold operation, the binary image has good uniformity as well as good shape information. To this end, the resulting threshold t for initial segmentation must satisfy the following relationship:
min(t1, t2)≦t≦max(t1, t2) (4)
In the sequence of
Tissue clustering step 1300 (
Referring to
To obtain the highly dense region, a threshold value is obtained by first eliminating the upper and lower 5% of values from histogram 1804; these are regions of the data that typically have high levels of noise content. This sets new MAX and MIN values at each end of the histogram. The value that is used for a threshold T is then computed as follows:
T=(MIN+0.75(MAX−MIN)
Dashed lines in
Following tissue clustering step 1300 in the sequence of
As shown in the block diagram of
In super-pixel size determination step 1401 of
wherein:
- Gm,σ Un-normalized Gaussian with mean m and standard deviation σ.
- ∥x′-x∥ Euclidean distance between x and x′.
Using this sequence, a super-pixel neighborhood, N, is defined for each pixel x∈X. The sequence for executing super-pixel size determination step 1401 to determine the radius of a circular neighborhood r(x), is as follows, using the example segmented image 1700 of
For each pixel x, a super-pixel is determined as follows:
-
- 1) Determine the largest circle that is centered at the pixel x within its respective region, whether fatty or dense.
- 2) Determine the radius r(x) of the circle from step 1 or generate a radius (distance) map in which the brightness of each pixel corresponds to this radius distance within its respective region.
Each super-pixel neighborhood N(x) is thus defined as a circular neighborhood with a radius of r(x). In the example of
Referring again to the localized segmentation sequence of
-
- 1) Assign a probability value of 1 to all the pixels within the highly dense region (image 1508 in
FIG. 2B ). - 2) Determine the mean mφ and standard deviation (STD) σφ of the highly dense region.
- 1) Assign a probability value of 1 to all the pixels within the highly dense region (image 1508 in
- mφ and σφ the mean and standard deviation of pixel intensities in the highly dense tissue region.
- 3) Calculate a weighted density probability Wφ(x) for each pixel outside the highly dense region as illustrated by the following equation
The next step in the sequence of
For any two neighboring pixels x1 and x2 (these could, alternately, be nearby pixels, separated by a distance of d) with super-pixels N(x1) and N(x2):
-
- 1) Determine the minimum radius min {r(N(x1)),r(N(x2))} of their respective circular or “super” neighborhoods.
- 2) Define two circles, each centered at one of the two points x1 and x2, each with a radius of min {r(N(x1)),r(N(x2))} obtained in step 1).
- 3) Calculate the intensity difference of corresponding points in the two circular neighborhoods, weighted by a Gaussian distribution as illustrated subsequently.
A component ψ measures homogeneity and indicates the level of intensity difference between the circular neighborhoods N(x1) and N(x2) by computing intensity differences of corresponding pixels between N(x1) and N(x2). Because the original radii of N(x1) and N(x2) may be different, the radii for both N′(x1) and N′(x2) are set equal to min {r(N(x1)),r(N(x2))}. Considering any two pixels x1′∈N′(x1) and x2′∈N′(x2) such that they represent the corresponding points within N′(x1) and N′(x2), that is, x1,i′ and x2,i′, the difference δ in intensity between the two corresponding points is computed:
δ(x′1,i, x′2,i)=|f(x′1,i)−f(x′2,i)| (7)
Then the weighted difference D between the two circular neighborhoods N′(x1) and N′(x2) is:
where
- mψ and σψ Expected mean and standard deviation of intensity differences between all pairs of adjacent pixels within initial dense tissue region, respectively.
- G is the Gaussian function.
The feature map can be computed as:
μk(x)=1/C√{square root over (Dψ(x)Wφ(x))}{square root over (Dψ(x)Wφ(x))} (9)
wherein Dψ gives the weighted difference for pixels in the initial dense tissue region. The feature map can be further normalized using 1/C.
Referring back to
Referring to
Embodiments of the present invention not only provide the automated calculations described, but also provide a viewer with the capability to enter and adjust threshold values used in this processing or select the best density segmentation from a set of pre-calculated density values.
As was described earlier with reference to
In one embodiment, the percent density calculation uses the segmented dense area from step 1516 (
Breast cancer risk (5-year, 10-year or lifetime risk) can be estimated using existing clinical risk models such the Gail model, familiar to those skilled in mammography risk assessment, based on age, family history, patient history and breast density. The calculated risk can be reported along with other information in the breast density report.
Similar to breast density as shown in
Embodiments of the present invention make it possible to provide a heightened level of automated risk management for patients having a density value above a threshold or having other characteristics that make it advisable to monitor density more closely. Embodiments of the present invention can be used for mammography images from any type of radiographic equipment, whether from scanned film, CR, or DR modalities. Because the method of the present invention is insensitive to absolute density differences, it can be readily used for patients having mammograms taken on film and taken using CR and DR media.
The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. While the methods of the present invention have been described with reference to mammography, these can also be applied for other types of tissue imaging where it is useful to distinguish and otherwise characterize tissue according to its relative density.
The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.
Parts List
- 36. Control console
- 38. Patient database
- 40. CAD system
- 42. Digitized image
- 44. Input image processor
- 46. Display
- 48. Risk modeling processor
- 50. Graphical user interface
- 52. Control
- 54. Display
- 56. Display
- 60. Dense region
- 62. Highly dense region
- 64a, 64b, 64c, 64d. Image
- 66. Image
- 1102. Segmentation step
- 1104. Skin line estimation step
- 1106. Computation step
- 1110. Test step
- 1114. Image
- 1200. Segmentation step
- 1300. Tissue clustering step
- 1400. Localized segmentation step
- 1401. Super-pixel size determination step
- 1404. Density probability map generation step
- 1406. Homogeneity map generation step
- 1500. Digitized mammography image
- 1502. Processed image
- 1504, 1506. Binary clustering results
- 1508. Overlaid image
- 1510. Density probability map
- 1512. Homogeneity map
- 1514. Feature map
- 1516. Overlay image
- 1700. Image
- 1710. Calculation step
- 1720. Final step
- 1800. Dense tissue region
- 1802. Seeding region
- 1804. Histogram
Claims
1. A method for assessing breast density, executed at least in part by a computer system, the method comprising:
- identifying breast tissue from the electronic image data for at least one mammographic image;
- performing an initial segmentation of fibroglandular tissue within the breast tissue according to at least one of gradient and uniformity data that is derived from the image data;
- generating a refined segmentation of the fibroglandular tissue within the breast tissue by refining the initial segmentation using a pixel clustering process;
- obtaining a localized segmentation from the refined segmentation by generating and combining a density probability mapping and a homogeneity mapping from the image data; and
- calculating a percent density value for the at least one image and storing the percent density value in a memory.
2. The method of claim 1 further comprising displaying the at least one image with detected fibroglandular tissue highlighted in a color.
3. The method of claim 1 wherein generating the refined segmentation comprises applying fuzzy c-means clustering.
4. The method of claim 1 wherein performing the initial segmentation comprises:
- generating a reduced-resolution grayscale image;
- identifying a first threshold in the reduced-resolution grayscale image according to a computed uniformity;
- identifying a second threshold in the reduced-resolution grayscale image according to a computed gradient; and
- calculating and applying a third threshold that lies between the first and second thresholds.
5. The method of claim 1 further comprising obtaining a threshold value entered by a viewer for conditioning the refined segmentation processing.
6. The method of claim 5 wherein obtaining the threshold value comprises obtaining a value from an on-screen control that is manipulated by the viewer.
7. The method of claim 1 further comprising displaying a plurality of calculated percent density values for a patient, arranged according to patient age.
8. The method of claim 1 further comprising graphically displaying one or more calculated percent density values for a patient, along with an indicator of relative risk for one or more of the displayed values.
9. The method of claim 1 further comprising providing a binary segmentation and calculated percent density value to a risk modeling program.
10. The method of claim 1 wherein obtaining a localized segmentation further comprises:
- generating a weighted density probability for one or more pixels;
- generating a homogeneity mapping for the one or more pixels;
- generating a feature map as a product of the weighted density probability and homogeneity mapping; and
- applying a threshold to the feature map to segment dense from the fatty tissue.
11. The method of claim 10 wherein generating the weighted density probability comprises:
- identifying a highly dense region in the initial segmentation and estimating one or more intensity distribution statistics within the identified highly dense region;
- assigning a probability of 1 to each pixel in the highly dense region; and
- calculating a probability value for each pixel outside the highly dense region by calculating a Gaussian weighted intensity value for the pixel.
12. The method of claim 10 wherein generating a homogeneity mapping comprises calculating Gaussian weighted intensity differences over equal-sized areas surrounding two nearby pixels.
13. A diagnostic system for mammography comprising:
- an input image processor that is responsive to stored instructions for obtaining a digital mammography image;
- a computer-aided diagnostic system that is responsive to stored instructions for performing an initial segmentation of fibroglandular tissue according to at least one of gradient and uniformity data derived from the image, for refining the initial segmentation according to pixel clustering, for processing the refined segmentation according to computed density probability and homogeneity mapping, and for calculating a percent density value;
- a memory operatively associated with the input image processor and storing the computed percent density value;
- a risk modeling processor in communication with the computer-aided diagnostic system for obtaining at least the computed percent density value; and
- a display operatively connected with the computer-aided diagnostic system and risk modeling processor for displaying at least the computed percent density value.
Type: Application
Filed: May 26, 2009
Publication Date: May 20, 2010
Inventors: Zhimin HUO (Pittsford, NY), Zhiqiang LAO (Newtown, PA)
Application Number: 12/471,675
International Classification: G06K 9/00 (20060101);